A light-emitting diode (LED) is a type of semiconductor device, specifically a p-n junction, which emits electromagnetic radiation upon the introduction of current thereto. Typically, a light-emitting diode comprises a semiconducting material that is a suitably chosen gallium-arsenic-phosphorus compound. By varying the ratio of phosphorus to arsenic, the wavelength of the light emitted by a light-emitting diode can be adjusted.
With the advancement of semiconductor materials and optics technology, light-emitting diodes are increasingly being used for illumination purposes. For instance, high brightness light-emitting diodes are currently being used in automotive signals, traffics lights and signs, large area displays, etc. In most of these applications, multiple light-emitting diodes are connected in an array structure so as to produce a high amount of lumens.
FIG. 1 illustrates a typical arrangement of light-emitting diodes 1 through m connected in series. Power supply source 4 delivers a high voltage signal to the light-emitting diodes via resistor R.sub.1, which controls the flow of current signal in the diodes. Light-emitting diodes which are connected in this fashion usually lead to a power supply source with a high level of efficiency and a low amount of thermal stresses.
Occasionally, a light-emitting diode may fail. The failure of a light-emitting diode may be either an open-circuit failure or a short-circuit failure. For instance, in short-circuit failure mode, light-emitting diode 2 acts as a short-circuit, allowing current to travel from light-emitting diode 1 to 3 through light-emitting diode 2 without generating a light. On the other hand, in open-circuit failure mode, light-emitting diode 2 acts as an open circuit, and as such causes the entire array illustrated in FIG. 1 to extinguish.
In order to address this situation, other arrangements of light-emitting diodes have been proposed. For instance, FIG. 2(a) illustrates another typical arrangement of light-emitting diodes which consists of multiple branches of light-emitting diodes such as 10, 20, 30 and 40 connected in parallel. Each branch comprises light-emitting diodes connected in series. For instance, branch 10 comprises light-emitting diodes 11 through n.sub.1 connected in series. Power supply source 14 provides a current signal to the light-emitting diodes via resistor R.sub.2.
Light-emitting diodes which are connected in this fashion have a higher level of reliability than light-emitting diodes which are connected according to the arrangement shown in FIG. 1. In open-circuit failure mode, the failure of a light-emitting diode in one branch causes all of the light-emitting diodes in that branch to extinguish, without significantly effecting the light-emitting diodes in the remaining branches. However, the fact that all of the light-emitting diodes in a particular branch are extinguished by an open-circuit failure of a single light-emitting diode is still an undesirable result. In short-circuit failure mode, the failure of a light-emitting diode in a first branch may cause that branch to have a higher current flow, as compared to the other branches. The increased current flow through a single branch may cause it to be illuminated at a different level than the light-emitting diodes in the remaining branches, which is also an undesirable result.
Still other arrangements of light-emitting diodes have been proposed in order to remedy this problem. For instance, FIG. 2(b) illustrates another typical arrangement of light-emitting diodes, as employed by a lighting system of the prior art. As in the arrangement shown in FIG. 2(a), FIG. 2(b) illustrates four branches of light-emitting diodes such as 50, 60, 70 and 80 connected in parallel. Each branch further comprises light-emitting diodes connected in series. For instance, branch 50 comprises light-emitting diodes 51 through n.sub.5 connected in series. Power supply source 54 provides current signals to the light-emitting diodes via resistor R.sub.3.
The arrangement shown in FIG. 2(b) further comprises shunts between adjacent branches of light-emitting diodes. For instance, shunt 55 is connected between light-emitting diodes 51 and 52 of branch 50 and between light-emitting diodes 61 and 6225 of branch 60. Similarly, shunt 75 is connected between light-emitting diodes 71 and 72 of branch 70 and between light-emitting diodes 81 and 82 of branch 80.
Light-emitting diodes which are connected in this fashion have a still higher level of reliability than light-emitting diodes which are connected according to the arrangements shown in either FIGS. 1 or 2(a). This follows because, in an open-circuit failure mode, an entire branch does not extinguish because of the failure of a single light-emitting diode in that branch. Instead, current flows via the shunts to bypass a failed light-emitting diode.
In the short-circuit failure mode, a light-emitting diode which fails has no voltage across it, thereby causing all of the current to flow through the branch having the failed light-emitting diode. For example, if light-emitting diode 51 short circuits, current will flow through the upper branch. Thus, in the arrangement shown in FIG. 2(b), when a single light-emitting diode short circuits, the corresponding light-emitting diodes 61, 71 and 81 in each of the other branches are also extinguished.
The arrangement shown in FIG. 2(b) also experiences other problems. For instance, in order to insure that all of the light-emitting diodes in the arrangement have the same brightness, the arrangement requires that parallel connected light-emitting diodes have matched forward voltage characteristics. For instance, light-emitting diodes 51, 61, 71 and 81, which are parallel connected, must have tightly matched forward voltage characteristics. Otherwise, the current signal flow through the light-emitting diodes will vary, resulting in the light-emitting diodes having dissimilar brightness.
In order to avoid this problem of varying brightness, the forward voltage characteristics of each light-emitting diode must be tested prior to its usage. In addition, sets of light-emitting diodes with similar voltage characteristics must be binned into tightly grouped sets (i.e.--sets of light-emitting diodes for which the forward voltage characteristics are nearly identical). The tightly grouped sets of light-emitting diodes must then be installed in a light-emitting diode arrangement parallel to each other. This binning process is costly, time-consuming and inefficient.
Therefore, there exists a need for an improved light-emitting diode arrangement which does not suffer from the problems of the prior art, as discussed above.